The phenomenon of an electrochemically induced spectroscopic change in a material, usually a change of color, is usually called electrochromism. Taking advantage of this phenomenon, it is possible to manufacture, for example, optical devices with variable transmission or reflection, in which the degree of reflection or transmission can be changed as a function of an applied voltage.
In such electrochromic elements, an electrochromic material with guest ions or electrons, fed into the electrochromic material by applying an electric potential, interacts, their interaction being influenced by incident electromagnetic radiation. Typical examples of such electrochromic materials are WO3 and MoO3, which, when applied in thin layers on a carrier, are almost colorless. If protons penetrate into such a layer, for example in the case of tungsten oxide (WO3), a reduction to blue tungsten bronze will take place, the intensity of the coloring being determined by the charge quantity that has flown in the layer.
Electrochromic elements manufactured on the basis of this principle can be used in different devices in which a purposeful variability of the reflection or transmission properties is desirable. Such devices can be used, for example, in windows and canopy tops, in particular for motor vehicles. Such applications are, however, particularly advantageous in the manufacture of antidazzle rear-view mirrors for motor vehicles, because it is one problem of such rear-view mirrors, which for good recognizability in daylight are usually designed for a particularly high reflectance, that at night, they can be very disturbing, in particular with regard to a possible dazzlement through following vehicles. Therefore, concerning the spectral distribution of the light emitted by the headlamps of motor vehicles, an efficient antidazzle behavior can be achieved in a particularly favorable way through suitable variation of the reflection properties of a rear-view mirror by coating such a mirror with a suitably chosen electrochromic material. On the one hand, in daylight, a particularly high reflectance can be maintained, and if required, i.e. for example after a sensorily detected incidence of light at night, the spectral reflection properties can purposefully be modified by applying a control voltage, thus achieving an antidazzle effect.
Especially in view of a possible use in rear-view mirrors of motor vehicles, it is desirable that such electrochromic elements achieve, with relatively short switching times, a particularly high so-called reflection stroke, namely the difference between the reflection maximum and the reflection minimum. In this way, the reduction of the dazzling effect which can be achieved by applying the control voltage, will be particularly high. Furthermore, electrochromic elements suitable for use in rear-view mirrors of motor vehicles should in general be designed for a particularly long service life, in view of the global service time and the number of switching cycles.
These requirements are fulfilled to a particularly high degree by an electrochromic element in so-called solid-state construction which is known, for example, from DE 196 40 515 A1. In this electrochromic element, a multilayer system is applied on a transparent carrier, for example on a glass substrate. The multilayer system comprises an ion storage layer applied directly onto the substrate, on which a transparent solid-state electrolyte layer is applied. On the latter, in turn, an electrochromic electrode layer is applied, which is covered by a reflector layer. Through this construction, in particular a high reliability and service-life stability can be achieved, which is required especially for applications in the field of motor vehicles. To guarantee at the same time also a marked antidazzle effect, a high reflection stroke is desirable especially for this type of electrochromic elements.